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Rails Associations: has_many & belongs_to

When creating a one to many relationship in rails tutorials refer to adding both has_many in the parent of the relationship (Customer) and belongs_to in the child (Orders). Why is this?

Specifically why do you need both? Is it not possible to only use has_many and not belongs_to? Is this just for clarity?


1 Answer

Naomi Freeman
Naomi Freeman
Treehouse Guest Teacher

Because each model/controller is just a table, and you need space in the table for the thing you're connecting to, for the relationship to exist. You do that by adding belongs_to and has_many. There are lots of other relationships you can define:


This is the important bit for this conversation: "In Rails, an association is a connection between two Active Record models. Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model belongs_to another, you instruct Rails to maintain Primary Key–Foreign Key information between instances of the two models, and you also get a number of utility methods added to your model."

A way I like to think of it is that every time you have a belongs_to or has_many, it gives your table either a key or a keyhole. Some keys are universal. Some are only good for one particular thing. And if the keyhole is 'polymorphic', it can accept a few different assigned keys.

I learned about relationships and how they appear in tables by doing Rails for Zombies. I love Treehouse! It is amazing! I love that learning one language is supported by learning other languages and design and so many important things. And relationships baffled me for a long time. I kept breaking my apps because I wasn't defining things right. I just couldn't see it.

Another way to see this is to check out your db - schema.rb. Just generate some random relationships and see how it changes it. You can also set this up by using references in the command line/terminal as you're generating things.

The schema file shows you what is in your table. Each line is a cell. The relationship needs a cell to exist in.

Hopefully one part of what I've said points you in the right direction to understanding relationships better :)